An antenna array comprises a plurality of elements, here, each including an antenna and an associated transceiver. The antennae are arranged in a manner and the transceivers driven such that radiation emanating from the array may be steered electrically. This is known as beamforming.
The difficulty in employing antenna arrays is that, in order avoid beamforming errors, the individual antennae each require stimulation at a correct radio frequency (RF) amplitude and phase. This requires an electrical path between transceivers and antenna elements to be equivalent.
Antenna array calibration is the process by which operation of the antenna elements at the correct amplitude and phase is established. For convenience, we categorise antenna array calibration schemes as either radiative (where the calibration is effected by an RF signal radiated in free space) or wired systems.
Typically, wired calibration schemes use internally generated calibration signals which are routed to an internal transceiver. FIG. 1 illustrates the basic principles of a wired calibration scheme. A transmitter calibration system 10 assumes operation at a single known and accurate frequency and comprises a first transceiver 12, a second transceiver 14 and a reference transceiver 16. Output power from the first transceiver 12 is coupled to the reference transceiver 16, which performs and records an amplitude and phase measurement. Output power from the second transceiver 14 is coupled to the reference transceiver 16, which performs an amplitude and phase measurement. A feedback path 18 is provided between an input of the second transceiver 14 and the reference transceiver 16 by which the amplitude and phase of the second transceiver 14 is modified until the measurement is equal to the recorded measurement of the first transceiver 12.
Thus, accurate amplitude and phase control may be attained provided that a calibration path, Cal Err 1, between the first transceiver 12 and the reference transceiver 16 and a calibration path, Cal Err 2 between the second transceiver 14 and the reference transceiver 16 are equal, i.e. Cal Err 1−Cal Err 2=0.
U.S. Pat. No. 6,339,339 discloses a similar scheme to that of FIG. 1. A common reference transceiver is employed to measure amplitude and phase of distributed antenna element transceivers of an antenna array, and to apply correction factors in order to calibrate transmission of the antenna array.
The drawback associated with wired calibration schemes is that they are impractical for use with large antenna arrays because of the need to connect all of the elements of the array back to a reference transceiver. Thus for large arrays, manual or radiative calibration techniques are often implemented.
Radiative calibration schemes involve the emission of RF energy into free space to ensure accurate element amplitude and phasing.
U.S. Pat. No. 5,657,023 discloses a radiative calibration scheme whereby mutual coupling between array elements is employed to derive an error signal. This involves an interleaved lattice of transceivers, which once phased form an operational antenna.
While practical in simplex systems, for example, radar systems, such a scheme has the disadvantage of requiring complex transceiver hardware in a full duplex communication system.
It is an object of the present invention to mitigate the disadvantages associated with the prior art.